Effects of Air Pollution on the Health of Older Adults during Physical Activities: Mapping Review

Atmospheric pollutants present environmental threats to health and have been investigated in different environments, such as highways, squares, parks, and gyms. These environments are frequented by older adults, who are considered fragile to the harmful impacts of pollution present in the air. The aim was to analyze the state of the art on the effects of air pollution on the health of older adults during physical activities (PAs) through a mapping review. The search was performed in PubMed, Web of Science, Scopus, and Cinahl databases until June 2022. Of the 10,109 studies initially identified, 58 met the inclusion criteria. The most investigated health outcome was cardiovascular disease, followed by respiratory outcomes. Particulate matter (PM2.5 and PM10), nitrogen dioxide (NO2), and ozone (O3) were the most investigated pollutants. Of the 75 health outcomes investigated, in 29, air pollution had harmful effects on the health of the older adults during the practice of PA, more frequently in cardiovascular diseases. In 25 outcomes, the beneficial effects of PA to the health of the older adults remained, despite exposure to high and low concentrations of pollutants, most often in terms of mental disorders. We conclude that poor air quality is a harmful factor for the health of older adults during the practice of PAs, more frequently in cardiovascular and respiratory diseases. On the other hand, for mental-health-related outcomes (depression and cognition), in most studies, the beneficial effects of PA in older adults were maintained, even after exposure to pollutants.


Introduction
Air pollution is one of the greatest environmental threats to human health. The main pollutants that present health risks are particulate matter (PM), ozone (O 3 ), nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), and carbon monoxide (CO) [1], which have been investigated in different environments, such as residential kitchens, highways, squares, parks, and gyms [1,2]. These environments are frequented by different populations, such as older adults. Today, this population is considered the most fragile to the harmful impacts of pollution present in the air [1] due to the entry of these pollutants deep into the lungs and bloodstreams, traveling through the organs of the human body, causing severe tissue and cell damage [1].
Global assessments suggest that air pollution is an all-cause mortality risk [3]. Among people aged 60 years and over, the greatest impacts on health and mortality levels are observed in individuals who already have chronic diseases such as asthma, chronic obstructive pulmonary disease (COPD), and heart disease [3]. Healthy practices can contribute to the reduction in reported diseases and mortality levels, such as physical activities (PAs);

Research Questions of the Review
On the basis of the work of Fernández-Sotos et al. [25], a key methodological aspect for a successful mapping review is the definition of research questions (RQs) to be answered. For this mapping review, we defined five RQs: -RQ1. How many articles have been published about the effects of air pollution on the health of older adults during physical activity? What are the characteristics of the studies? What is the geographical distribution of the research carried out? -RQ2. What were the most commonly investigated health-related outcomes? -RQ3. Which air and environmental pollutants were the most commonly investigated? -RQ4. Which types of PA were most investigated? Which ones presented greater health risks and which ones presented greater benefits when older adults were exposed to pollutants during PA practice? Which atmospheric and environmental pollutants compromised the health of older adults during PA practices? -RQ5. What are the effects of air pollution on health outcomes during PA in older adults?

Guidelines
This mapping review was carried out to analyze and summarize the effects of air pollution on the health of older adults during PA. The work was based on the recommendations for systematic mapping in environmental sciences by James et al. [26].
Three filters were applied in this mapping review to gather (research procedure), select (inclusion and exclusion criteria), and extract (data extraction) relevant information from the literature [27]. The steps described in the Template for a Mapping Study protocol [28] were followed (see Table S1).

Study Quality Assessment
To investigate the risk of bias in the studies, we used the Mixed Methods Appraisal Tool (MMAT), version 2018 [30]. The reliability and validity of the new version of the MMAT have been confirmed [30,31]. The MMAT is designed for the appraisal of mixed study reviews, which include studies with qualitative, quantitative, and mixed methods. To assess the quality of studies, the MMAT contains two initial screening questions for all study types, followed by five questions for each of five possible types of study design. We used the same evaluation criteria with this tool adapted from May et al. [32]. The "Can't tell" response category indicates that the article does not report information appropriate to answer "Yes" or "No", or that it reports unclear information related to the criterion. "Yes" response categories indicate that the article reports appropriate information for the criterion, while "No" response categories indicate that the article does not report appropriate information for the criterion.

Literature Search Results
The literature search identified 10,109 relevant articles. Of these, 80 full articles were evaluated for eligibility and 58 studies were selected for synthesis ( Figure 1).

Literature Search Results
The literature search identified 10,109 relevant articles. Of these, 80 full articles were evaluated for eligibility and 58 studies were selected for synthesis ( Figure 1). Below, the five research questions are answered on the basis of the synthesis of the analyses.
RQ1. How many articles have been published about the effects of air pollution on the health of older adults during physical activity? What are the characteristics of the studies? What is the geographical distribution of the research carried out?
The oldest study was published in 1987 [33], with a subsequent 58 published studies that investigated the topic by the year 2022 (see Table S3 of the Supplementary Material for a complete list of the selected studies). The period with the highest number of publications was between 2017 and 2019, with 13 studies, and 2020 to 2022, with 18 studies (Figure 2). The study with the longest follow-up period and duration was that of Elliot et al. [34], which lasted from 1988 to 2008. Below, the five research questions are answered on the basis of the synthesis of the analyses. RQ1. How many articles have been published about the effects of air pollution on the health of older adults during physical activity? What are the characteristics of the studies? What is the geographical distribution of the research carried out?
The oldest study was published in 1987 [33], with a subsequent 58 published studies that investigated the topic by the year 2022 (see Table S3 of the Supplementary Material for a complete list of the selected studies). The period with the highest number of publications was between 2017 and 2019, with 13 studies, and 2020 to 2022, with 18 studies (Figure 2). The study with the longest follow-up period and duration was that of Elliot et al. [34], which lasted from 1988 to 2008. The predominant countries that carried out the research were the USA, with 25 studies, followed by China with 10, and Canada with 3. Brazil, Chile, the Netherlands, Finland, England, Denmark, Belgium, and Korea performed two studies each, and Switzerland, Taiwan, United Kingdom, and Australia one study each ( Figure 3).  The predominant countries that carried out the research were the USA, with 25 studies, followed by China with 10, and Canada with 3. Brazil, Chile, the Netherlands, Finland, England, Denmark, Belgium, and Korea performed two studies each, and Switzerland, Taiwan, United Kingdom, and Australia one study each ( Figure 3).  The predominant countries that carried out the research were the USA, with 25 studies, followed by China with 10, and Canada with 3. Brazil, Chile, the Netherlands, Finland, England, Denmark, Belgium, and Korea performed two studies each, and Switzerland, Taiwan, United Kingdom, and Australia one study each ( Figure 3).   Of the 58 included studies, 20 had a longitudinal methodological design and 15 carried out a longitudinal experimental model. The remainder were classified as crosssectional (six studies); randomized controlled trials (5); pre-and post-analysis (3); longitudinal prospective cohort studies (2); repeated measures studies (2); experimental (2); and one each of cross-sectional retrospective, longitudinal prospective, and quasi-experimental prospective.
The studies included a total of 1,653,411 participants. The study with the largest sample was by Kim et al. [35] with 1,259,871 participants, in a longitudinal study. The studies with the smallest number of participants were those of Babb [36] and Drechsler-Parks [37], both experimental studies. The majority of the studies had a sample composed of men and women, totaling 51 studies. In three studies, only men participated, and in another three, only women; one study did not report these data. The average age of the participants varied, with the study with the youngest sample being by Huang et al. [38], aged 50 years or older. The study with the oldest sample was carried out by Bagheri et al. [39], including participants aged between 65 and 90 years or older. In the majority of studies, 28 participants had a mean age of 64 years (age range from 60 to 79 years).
Of the included studies, 23 involved older people without specificity in relation to health; 8 included healthy older adults and non-smokers; 4 only healthy older people, without specifying tobacco use or not; and 3 included non-smoking older people, not necessarily healthy. Some studies included older adults with specific diagnoses, six with a diagnosis of coronary artery disease (CAD) and non-smokers, one study with a diagnosis of CAD (without specifying tobacco use or not), and three studies included participants with a diagnosis of COPD. Only the study by Babb et al. [40] exclusively investigated older people who regularly exercise.
RQ2. What were the most commonly investigated health-related outcomes? Several health outcomes were investigated. The most common were cardiovascular disease outcomes, investigated in 14 studies in isolation and in another 10 together with other outcomes. The outcomes related to cardiovascular diseases mainly included blood pressure, a decline in the ST segment on the electrocardiogram (associated with myocardial ischemia), and heart rate. The second most investigated outcome was respiratory, in 19 studies (9 studies exclusively respiratory outcomes and another 10 combined). Among these outcomes, pulmonary function and peripheral oxygenation level stood out.
Outcomes of mental disorders were evaluated in 11 studies, with depression and cognition being the most present. Overall health outcomes were evaluated in nine publications, including outcomes such as upper and lower limb muscle strength, hand grip strength, sedentary behavior, and time outdoors. Five articles reported health records of older adults, including mortality and hospitalization data. Only three articles reported outcomes considered to be chronic diseases, in these cases, diabetes. Two articles explored other diseases, with outcomes of oxidative stress markers and liver function levels. No studies selected in the mapping review investigated cancer.
RQ3. Which air and environmental pollutants were most commonly investigated? What is the relationship between health outcomes and air pollutants during PA in older adults?
The investigated environments were classified into two types: internal or external. External environments, such as squares and parks, were present in 42 studies. Indoor environments were used in 15 studies, including research laboratories [41], with a controlled level of exposure to the pollutant, such as pollutant gases, except for the study by Gong et al. [42] who also controlled particle level. Regarding the analyzed area, 43 studies investigated urban areas, 4 investigated rural areas (only one exclusively), and 1 investigated commercial areas. These data were not reported in 15 studies.
Considering the class of pollutants, fine particles were the most widely investigated among the analyzed studies; PM 2.5 was analyzed in 35 studies, and in 10 of these, it was the only pollutant. Among fine particles, PM 10 was analyzed in 12 studies, PM 1.0 in 2, and PM 4.0 and PM coarse in 1 study each; three studies analyzed the level of fine particles, and two studies the total number of particles.
The second most investigated class was pollutant gases, with the most frequently investigated being NO 2 and O 3 in 24 studies each, followed by CO in 14 studies, SO 2 in 12 studies, and CO 2 and/He-O 2 in two studies. Other substances and biomasses were investigated in the fewest studies; NO x was evaluated in four publications, black carbon in five, organic carbon in three, and elemental carbon in two studies.
Regarding the results of health outcomes and pollutants during PA in older adults, it can be seen that the most investigated pollutants were PM 2 . 5 and PM 10 , present in all health outcomes. The most commonly investigated relationship between air pollutants and health outcomes was the effects of PM 2.5 on cardiovascular disease (18 studies). Table 3 shows the number of studies according to air pollutants and the health outcomes of the older adults during PA. RQ4. Which PAs were most commonly investigated? Which presented greater health benefits, and which presented greater risks when the older adults were exposed to pollutants during PA practice? Which atmospheric and environmental pollutants compromised the health of the older adults during PA practices?
Of the 58 articles that entered the mapping review, 26 of the authors did not report the PA that the older adults performed. Walking was investigated in 17 studies, followed by cycle ergometer exercise in 9 studies. Varied PA, such as playing ball, walking uphill, swimming, cycling, running, slow walking, stretching, and traditional Chinese exercises were investigated in seven studies, cycling in three studies, and the practice of resistance training with weights in only one study. Twenty-nine studies carried out specific programs or protocols in the investigated PA.
In 20 studies, PA was measured from the time and total days (minutes/week) performed, in 15 through various components such as distance, telemetry, time in minutes, heart rate, accelerometer, speed by GPS, electrocardiogram, arterial saturation of oxygen by pulse oximetry, blood pressure, perceived exertion rate, ventilation rate by plethysmography, and gas exchange measurements. In nine studies, PA was measured from the ventilatory volume per minute through the respiratory rate and in another four through the intensity of each type of PA attributed to the standard metabolic equivalent of the task value (MET-h/wk). In two studies, arterial oxygen saturation was measured by pulse oximetry and intensity measurement through actigraph, and a multisensory activity monitor was used for the average steps per day. In two studies, only one questionnaire was used to gather information on whether or not the participants performed PA, one study each used the one-repetition maximum strength test or 6 min walk test to measure PA, and only one did not report the way that PA was measured.
The following tables show the PAs, sample characteristics (sex and age), pollutants, health outcomes, and findings on benefits (grouped as negative) ( Table 4); risks (grouped as positive) ( Table 5); and those which presented limited interpretation or evidence (grouped as ambiguous) ( Table 6) of PA on health outcomes of older adults during exposure to polluted environments.
Of the 58 studies investigated, in 18, the PA showed benefits to the health of the older adults during exposure to pollutants. The most frequently reported PAs were PA in general (four studies), followed by PA in leisure, leisure and transport, moderate to vigorous, and slow walking accompanied by other activities in three studies each. Walking only, a controlled exercise protocol (stretching, treadmill walking, park walking), incremental cycle ergometer test, and resistance training were employed in only one study each. Urban environments were the most frequently (12 studies) investigated during these PA, followed by five studies that did not report the environment and one with urban and rural environments. With respect to open and closed environments, 16 studies were carried out in open environments, 1 in a closed environment, and 1 in a mixed (open and closed) environment. As for air pollutants that did not compromise the health of older adults during PA practices, PM 2.5 was the most frequent, in 14 studies, followed by NO 2 and PM 10 in seven studies (Table 4).
In 22 studies, the exposure of the older adults to pollutants presented health risks during PA, more frequently to PA in general (seven studies), followed by submaximal exercise tests on a cycle ergometer and exercise bike in five studies, walking outdoors and PA outdoors in three, and cycle ergometer and incremental exercise on a treadmill in two studies each. The environments by location investigated during these PA were urban in 19 studies, rural in 1 study, and not reported in 2 studies. Open environments were investigated in 16 studies and closed environments in 6 studies. PM 2.5 was the air pollutant that presented the greatest risk to older adults during PA in 13 studies, followed by O 3 in 11, CO in 8, and NO 2 in 6; other pollutants and information about these studies can be found in Table 5.
RQ5. What are the effects of air pollution on health outcomes during PA in older adults? Regarding the health outcomes investigated in the older adults, for the effects of air pollution during PA, of the 75 outcomes investigated, air pollution had harmful effects on the health of the older adults during PA practice in 29, more frequently in cardiovascular diseases. On the other hand, in 25 outcomes, the beneficial effects of PA on the health of the older adults remained despite exposure to high and low concentrations of pollutants, more frequently in mental disorders; and in 21, the evidence was limited or inconclusive, especially in respiratory diseases. Figure 4 presents the frequency according to the health outcomes investigated during PA in the older adults and the effects of pollution. Table 7 presents the results of the quality assessment. In total, 14 of the 58 studies that entered the present review did not have any negative classification, indicating high quality. A further 11 articles had only one negative classification and 14 two negative ratings, presenting medium-to-moderate quality. However, 13 studies presented between four and five negative classifications, indicating low quality. This factor occurred mainly due to the lack of methodological detail in the studies.

Open close Urban
Indoor walking was shown to provide better cognitive and physiological outcomes than outdoor exercise. Open Urban The benefits of PA remain in people exposed to various levels of PM 2,5 , and it is recommended for people living in relatively polluted areas.

Open Urban
The level of PA modified the effect of PM 2.5 on the body composition of the older adults, lower levels of physical activity were associated with greater loss of muscle mass, and increase in fat mass was related to exposure to the pollutant. Open Urban PA can help protect against air-pollution-induced mortality, as people who practice PA were at a lower risk of mortality from ambient air pollution than inactive people.    Moderate-to-strenuous exertion (daily and antigraph) was correlated with increased systolic and diastolic BP. AF was not associated with attenuation of the effects of pollution.
There was a greater association between pollutants and increased PA in participants with obesity. Open Urban The fraction of PM from combustion processes, notably from traffic, may be responsible for the effect observed in exercise-induced ischemic heart disease on days with high air pollution. Older adult volunteers without clinically cardiopulmonary disease were more vulnerable to factors related to autonomic control of the heart, inflammatory mediators, and circulating blood clotting factors when exposed to the fine effects of PM immediately after performing intermittent exercise than people of a similar age with COPD. There was an increased risk of changes in lung function at close to ambient levels after exposure to O 3 during exercise in healthy older adult subjects.   We found a direct and negative association of residential green with hypertension prevalence for rural participants, but not for urban participants. The association of green with hypertension was completely mediated by PM 2.5 concentrations (without any mediating effect of physical activity and BMI) in urban areas. In contrast, the association was mediated by concentrations of PM 2.5 , PA and others not observed in rural areas. Walking on a green route (lower degree of pollution) provided more benefits for the well-being of the older adults, while walking on a gray area (higher degree of pollution) increased a physiological factor related to stress.  The high ventilation rate of cycling resulted in the highest inhaled dose of NO 2 , when compared to other modes of transport such as walking, driving and stationary. However, the daily proportion of cycling activity was relatively low and the influence of this activity on the total amount of inhaled NO 2 is small. There were no significant differences between the responses to the three O 3 exposures during walking on a motorized treadmill. All three O 3 exposures induced small, statistically significant decreases in forced vital capacity and forced expiratory volume, but no significant changes in forced expiratory flow rate between 25 and 75%. The data suggest possible saturation of the pulmonary response mechanism with a low degree of functional impairment in older men.       [36] No Can't tell Can't tell Can't tell Can't tell Drechsler-Parks (1995) [37] No Yes Can't tell Can't tell Can't tell  Table 7.
pollution during PA, of the 75 outcomes investigated, air pollution had harmful effects on the health of the older adults during PA practice in 29, more frequently in cardiovascular diseases. On the other hand, in 25 outcomes, the beneficial effects of PA on the health of the older adults remained despite exposure to high and low concentrations of pollutants, more frequently in mental disorders; and in 21, the evidence was limited or inconclusive, especially in respiratory diseases. Figure 4 presents the frequency according to the health outcomes investigated during PA in the older adults and the effects of pollution. Limited or inconclusive evidence on the health of elderly people exposed to pollution during PA

Discussion
This is the first mapping review carried out on the effects of air pollution on the health of older adults during PA practices, considering articles with different designs. Air pollution is one of the greatest environmental threats to human health [3], while PA practices present several health benefits [90]. Harmful, beneficial, and inconclusive effects on the health outcomes of older adults during PA and exposure to air pollution were observed in the studies; thus, we sought to develop this discussion on the basis of these results and relate it to the various findings on this topic in the literature.
The benefits of PA and exercise for older adults are widely known and documented in several health outcomes, such as cardiovascular, respiratory, cognitive, psychological, and quality of life [91]. In the present review, 16 studies showed that the benefits of PA were maintained even when considering exposure to air pollutants. Mental disorders was the outcome with the highest amount of evidence of benefits during exposure to pollutants, without studies pointing out risks. Currently, the deleterious effects of air pollution on mental and cognitive health are well established, for example, it is known that exposure to PM 2.5 and NO 2 is a risk factor for the development of Alzheimer's disease and that increased exposure to PM 2.5 , NO 2 , and O 3 is related to the development of depression in older adults [92][93][94][95][96]. In contrast, exercise and PA in general produce effects that decrease depression and improve mood in older adults [97,98], as well as being a recognized way to treat Alzheimer's disease [5]. Thus, the findings demonstrate the importance of PA in the mental health of the older population.
PA has been recommended for older people with cardiovascular disease, and studies are well established on this topic in the literature [99]. However, on the basis of the articles included investigating cardiovascular diseases, we observed that the practice of PA during exposure to pollutants can further compromise the conditions of this disease. On the other hand, it is important to note that of the 15 studies that reported these harmful results, in 8 of them, the participants already had cardiovascular diseases [42,[63][64][65][66] or a high prevalence of developing cardiovascular diseases [70,72], while in the 5 studies that reported beneficial results, the sample was made up of healthy older adults, without a specific diagnosis. This demonstrates that PA even under exposure to air pollutants can provide a mechanism for preventing and maintaining cardiovascular health in older adults, as shown in the studies by Elliott et al. [34], Endes et al. [53], and Kubesch et al. [52] who demonstrated the benefits of PA in relation to the risk of myocardial infarction, stroke, and arterial stiffness.
The literature already points out that the adverse effects of air pollution can potentiate comorbidities, such as preexisting cardiovascular diseases, even attenuating the beneficial effects of PA on practitioners [100]. However, the population-based study by Kim et al. [35] (189,771 participants, more than 50% of whom were older adults) with a two-year follow-up found that moderate-to-vigorous PA ≥5 times/week was significantly associated with a decrease in the risk of cardiovascular disease, even when exposed to low and high levels of PM 10. These results were also maintained during exposure to high and low levels of PM 2,5 , except for coronary heart disease [35].
However, air pollution had a negative impact on healthy older people during PA, both in cardiovascular and respiratory health variables, presenting acute adverse symptoms in the study by Stieb et al. [58,60], including pulmonary vascular and inflammatory changes [68], as well as increased heart rate [61]. These adverse symptoms found in older adults can be justified by the direct inhalation of environmental gases, or by oxidative stress due to exposure to various pollutants [61].
Of the studies that only investigated respiratory diseases, the results are divided as to the risks of the practice of PA by older adults in environments with air pollution, presenting limited or inconclusive evidence in eight studies each. Despite this result, two studies showed benefits from the practice despite the exposure [40,50], both being longitudinal experimental studies. These results are controversial considering those published in the literature, primarily because most of the participants in these two studies are older women [101,102], and secondly because high ventilation induced by PA increases the exposure and rate of particle deposition in the lungs [103,104], which potentiates harmful effects on health [12]. In addition, high-quality evidence from a recent systematic review of randomized controlled trials indicated that moderate-to-vigorous PA practice during exposure to pollutants has adverse effects on lung function and related symptoms [105].
Considering health records, we observed that the majority of studies that investigated this outcome maintained the beneficial effects of PA on the health of the older adults during exposure to pollutants [34,47,54,56]. This is in contrast to recent literature results and the WHO itself, since exposure to pollutants is strongly associated with the risk of disease development and mortality prevalence, for example, long-term exposure to PM 2.5 or NO 2 is associated with stroke, NO 2 is associated with higher cases of atrial fibrillation, and exposure to O 3 increases the risk of hospitalizations for pneumonia and COPD exacerbations [3,106,107]. On the other hand, PA practice promotes the opposite outcome. The study by Cunningham et al. [91] found a reduction in the risk of CAD in men and a 40% reduction in mortality from cardiovascular diseases in moderate-to-vigorous PA practitioners. In the current review, PA practice provided lower cardiovascular and respiratory mortality and mortality in general [47,54,56], even with exposure to air pollutants. In the study by Yu et al. [59], it was found that exposure to PM 2,5 contributed to the reduction in PA levels in older adults. In addition, a negative association was found between PA score and leisure time with days of hospital care in one year. Although the findings do not indicate cause and effect, this demonstrates that air pollution indirectly affects the health of older adults, as it reduces the practice of PA, which is associated with hospitalization.
Few studies have analyzed the effects of exposure to pollution during PA on chronic or other diseases, in addition to respiratory, cardiovascular, and mental disorders in older adults. Of the four studies that analyzed these effects, none presented risks to the health of the older adults. The study by Kim et al. [35] points out that moderate-to-vigorous PA decreases the risk of developing diabetes, even with exposure to pollutants, which is extremely important, as currently 25% of older adults in the USA have a diagnosis of diabetes and 48% are pre-diabetic [108].
The study by Cassilhas et al. [41] reported an important comparison of PA in an open environment versus a closed environment, and although the practice of PA in an open environment does not produce risks, when the same activity is performed in a closed environment, it has greater benefits for other diseases and cognition of older adults. Unfortunately, none of the studies included in the review analyzed the outcome of cancer.
However, in the current literature, there is strong evidence that pollution and high levels of PM increase the incidence and mortality of lung cancer, although for other cancers, the evidence is limited [109].

Limitations and Future Studies
Although 53 studies have been published since 1987, focusing on the last few years, an effort by researchers is necessary to produce knowledge about the effects of air pollution on the health of older adults during PA, because despite the relevant number of published studies, the results are divergent, limiting the development of specific conclusions. Most of the studies also did not report the lifestyle, habits, where they worked, and the environments where the participants lived in recent years and decades, which, if they were environments with high levels of pollution, could affect the results of the research in which they participated. The focus of further studies should be on evaluating different models and scenarios, combining types of PA in different environments in relation to different levels of air pollution, and calculating possible risks to the health of older adults. Knowing the limits older adults can be exposed to when they decide to practice indoor and outdoor PA is essential, in addition to developing safety recommendations for this population.

Innovations, Study Strengths, and Practical Applications
To our knowledge, after an extensive search in international databases, the current mapping review is the first to be developed in the literature. In addition to this innovative and relevant character, considering that the older population is highly sensitive to air pollution, this review is even more relevant because this sensitivity is enhanced when they are practicing PA.
In the current scenario of the COVID-19 pandemic, the need for studies with these outcomes increases even more [110,111]. The idea is to provide greater safety and evidence of the risks and benefits to older adults, in different scenarios of pollution, environments, and PA.
Safety recommendations should be analyzed and written for the practice of PA by older adults in different environments, on the basis of the data of this mapping review and other systematic reviews under development. The results and our analyses and conclusions allow several practical applications, including greater attention to the previous health conditions of older adults in relation to PA practice environments, taking into account previous diseases, for example, heart diseases, in relation to environments polluted with nanoparticles from the combustion of organic material, which can significantly increase cardiorespiratory health risks.
On the basis of the recommendations of the studies included in this mapping review, we stratified relevant information for public policymakers, health professionals, researchers, and practitioners of physical activities, in order to provide guidance on the topic addressed in this review, and from there, to develop new public policies based on the evidence presented, as well as identifying gaps to be addressed in further research. Guidelines for older practitioners of physical activities in environments with levels of pollution were highlighted. On the basis of these points, care and observations that need to be taken into account for PA practice are presented in Table 8. Table 8. Recommendations for policymakers, health professionals, researchers, and practitioners of physical activities.

Politicians, Health Professionals, and Researchers Practitioners of Physical Activities
• More research on the combined effects of PMs and PA on diabetes is urgently needed.
• It is suggested that aerobic exercise in the open air be avoided when there are high levels of PM, so as not to affect the positive effects of the practice, in the levels of neuro factors derived from the brain.

•
We suggest that urban planners design green spaces in neighborhoods where older adults can nurture and communicate with each other. They can also plant trees along sidewalks to provide a desirable walking environment for older adults and recreational infrastructure under shaded trees to increase community comfort among older adults. Finally, green spaces can be developed in communities with a high proportion of senior citizens, especially for those belonging to low-income groups.
• It is recommended that physical activities be carried out in places with green spaces, as this encourages the practice, mitigates air pollution, increases social cohesion in localities/neighborhoods, and improves satisfaction with life; • Studies with a window of exposure to PM 2.5 greater than or equal to 180 days are recommended on the effects on depressive symptoms.
• Moderate-to-vigorous physical activity (MVPA) is recommended for older adults wherever they live, although practicing MVPA in an environment that considers air pollutant reduction is important to maximize the health benefits of MVPA in diabetes.
• Experimental studies are needed to clarify the specific mechanisms among older people with low levels of physical activity and vitamin D deficiency, exposed to high levels of PM 2.5 , presenting high chances of depression.
• For countries or regions with high pollution, especially those cities with poor sunlight or a high prevalence of vitamin D deficiency, additional vitamin D supplementation and physical activity are recommended. This can help to mitigate the adverse effects of exposure to PM 2.5 on health conditions and may hinder the occurrence of major depressive disorder.
• Policies and strategies are needed to reduce O 3 exposure to ensure that the health benefits of physical activity are not diminished by higher O 3 levels in susceptible populations, such as older Hispanics and type 2 diabetics.

•
The use of active transport is recommended to reduce sedentary levels, such as bicycles and other modes; however, it is recommended only when the concentration of air pollution is reduced in order to limit negative impacts on health. • Structural change policies to reduce levels of exposure to pollutants in built and social environments, such as parks with walking paths or recreational and sporting infrastructure environments, are potential opportunities to delay or reduce the risk of dementia.
• Physical activity is recommended for people with dementia or Alzheimer's disease who live in relatively polluted areas.
• Longitudinal studies are needed to provide a cause-and-effect association between hypertension and the practice of physical activity in green spaces in urban areas, mediated by air pollution, which can help policymakers and professionals to conduct effective interventions to prevent and control the prevalence of hypertension and the burden of concomitant diseases.
• Avoiding physical activity when exposed to air pollution at chronic levels, as it results in mood swings and increases the risk of depression or anxiety disorders. However, it is suggested that when levels are medium/low, walking more than 9000 steps per day be carried out, mitigating the harmful effects of air pollution.
• Future studies are needed to confirm the impacts of pollutants on indoor and outdoor exercise.
• Older people, when performing physical exercises in external environments, when possible, should be monitored by professionals with greater attention to climatic conditions and pollution, suggesting the use of a portable monoximeter as a monitoring instrument.

Politicians, Health Professionals, and Researchers Practitioners of Physical Activities
• It is necessary that the development of outdoor physical activity guidelines and emergency response plans for heavy air pollution be discussed.

•
We recommend, especially for the most susceptible populations, that they avoid increasing the absorption of atmospheric pollutants through simple measures, such as avoiding active displacement or exercise along busy main roads, as well as opting for less polluted alternatives, such as secondary roads, parks with green areas etc.
• Future studies are needed to validate whether the positive health effects of moderate-to-vigorous physical activity (MVPA) outweigh the potential harmful effects due to increased exposure to air pollution during MVPA.
• It is suggested that healthy people, as well as those with chronic cardiorespiratory disorders, should minimize walking on streets with high levels of pollution, as this reduces or even reverses the cardiorespiratory benefits of exercise. Instead, walking exercise should be enjoyed in urban green areas away from high-density traffic. • More research is needed to expand knowledge and understanding of the protective effects that planned physical activity have on cognitive function and cardiovascular adaptations in the presence of high concentrations of particulate matter in the atmosphere.
• It is suggested that older people living in rural areas may benefit from reducing outdoor activity when air pollution levels are particularly high, in order to reduce acute adverse cardiorespiratory effects.
• Additional research is recommended with the aim of identifying points on the Air Quality Health Index (AQHI) scale to provide protective advice that optimizes the balance between reducing outdoor activity when needed, in order to reduce short-term health risks of air pollution, preserving the long-term benefits of outdoor physical activity.
• Current environmental levels of air pollution along busy streets are unacceptable and need to be controlled. It is important to impose policies and measures that can reduce traffic pollution so that each individual can enjoy the benefits of physical activity for health.
• Policy interventions are needed to reduce the level of air pollution in China's urban areas.
• New studies with longitudinal designs and conducted in broader areas of exposure to PM 2.5 are needed to verify the relationship between physical activity and body composition, determining the effects on muscle function.
• Additional research in large prospective cohorts is needed to assess whether the observed effect modification (moderate-to-vigorous physical activity confers a protective effect against the adverse vascular effects of air pollution in low-pollution environments) translates to high-pollution environments in megacities in low-and middle-income countries.
• New studies that assess health impacts through epidemiological studies need to incorporate individual travel behavior and physical activity to measure the inhaled dose of air pollution. This can be performed accurately using GPS and accelerometer data.
• Training physical education professionals to care for the practice of exercises with older adults in polluted environments can minimize the health risks of this population.

Conclusions
On the basis of the current mapping review of 58 studies from four databases to analyze the state of the art on the effects of air pollution on the health of older adults during physical activities, we concluded that air pollution and poor air quality are harmful factors to the health of older adults during the practice of PAs. A significant number of studies show that air pollution has harmful effects on the health of older adults during the practice of PA, with greater frequencies in cardiovascular and respiratory diseases. On the other hand, in mental-health-related outcomes (depression and cognition), the beneficial effects of PA in older adults were maintained even during exposure to pollutants at their highest frequency. The most investigated pollutants during PA in the older adults were PM 2.5 and PM 10 , and the most commonly investigated relationship between air pollutants and health outcomes was on the effects of PM 2.5 in cardiovascular disease. Walking was the most widely investigated PA, followed by cycle ergometer exercise, and PA such as playing ball, walking uphill, swimming, cycling, running, slow walking, stretching, and traditional Chinese exercises, in addition to resistance training with weights. Future studies should combine different types of PA in several environments, considering both indoor and outdoor pollutants to calculate risks to the health of older adults.
Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/ijerph20043506/s1. Table S1: Template for a Mapping Study Protocol. Table S2: Literature search strategy on PubMed. Table S3: Titles of included reviews (58).

Conflicts of Interest:
The authors declare no conflict of interest.